Pure-titanium sheets are used as a starting material of various products such as heat exchangers, welded tubes, and a two-wheeled exhaust system including mufflers. In recent years, there is an increasing need to strengthen titanium sheets to reduce the wall thicknesses and the weight of these products. There is also a demand of keeping both high strength and the workability of pure-titanium sheets as before. Among others, pure titaniums that have excellent workability are used especially for a starting material of a plate-type heat exchanger (hereinafter, will be referred to as a “plate heat exchanger”) because the starting material is to be press-molded into a complex shape.
To enhance a heat-exchanging efficiency required for a plate heat exchanger, the reduction of wall thickness is needed. Since the wall thickness reduction decreases workability and pressure resistance performance, it is necessary to secure sufficient workability and enhance strength. Thus, in order to obtain more excellent strength-workability balance than that of a normal pure titanium, studies are underway regarding the optimization of the content of O, the content of Fe, and the like, and grain size control.
For example, Patent Document 1 discloses a pure-titanium plate having an average grain size of 30 μm or larger. However, pure titaniums are inferior in strength.
Hence, Patent Document 2 discloses a titanium alloy plate that contains amounts of O and Fe as a β stabilizing element, and including α phase of the average grain size of which is 10 μm or smaller. Patent Document 3 discloses a titanium alloy sheet that contains decreased amounts of Fe and O, and contains Cu to cause Ti2Cu phase to precipitate, so as to suppress the growth of crystal grain sizes by the pinning effect, and that has an average grain size of 12 μm or smaller. Patent Document 4 discloses a titanium alloy that contains Cu, and has a decreased content of κ.
According to these documents, use is made of the fact that, when a titanium contains alloying elements in large quantities, crystal grains are made fine, and the titanium is likely to have high strength, and further, workability is secured by decreasing the content of O and the content of Fe. However, the techniques disclosed in these documents fail to show high strength while keeping sufficient workability to the extent that can meet the demands of recent years.
Meanwhile, in contrast to these documents, techniques to coarsen crystal grains while containing alloying elements are studied.
For example, Patent Document 5 discloses a titanium alloy used for a cathode electrode for producing electrolytic copper foil, the titanium alloy having a chemical composition that contains Cu and Ni, and being annealed at a temperature within a range of 600 to 850° C. to have a crystal grain size adjusted to 5 to 50 μm, and discloses a method for producing the titanium alloy. Patent Document 6 discloses a titanium plate for a drum for producing electrolytic Cu foil that has a chemical composition containing Cu and Cr, and small amounts of Fe and O, and discloses a method for producing the titanium plate. This document describes an example in which annealing is performed at 630 to 870° C.
Patent Documents 7 and 8 disclose techniques that prepare a titanium having a chemical composition containing Si and Al, decrease the rolling reduction of cold rolling to 20% or lower, and increase annealing temperature to 825° C. or higher and a β transformation point or lower, which is a higher temperature condition, so as to make an average grain size 15 μm or larger.